1. Field of the Invention
The present invention relates to orthopaedic implants, and, more particularly, to glenoid implants having an articulating surface.
2. Description of the Related Art
An orthopaedic implant for implantation at the shoulder joint typically includes a glenoid implant and a mating humeral implant. The glenoid implant is attached to a prepared glenoid or scapula, and the humeral implant is attached to a prepared humerus. The humeral implant usually includes a ball at an end thereof which engages and moves relative to an articulating surface in the form of a concave surface formed in the glenoid implant. The ligaments and muscles of the body surrounding such an orthopaedic implant maintain the humeral implant against the glenoid implant, while at the same time allowing relative movement therebetween.
A glenoid implant is typically formed with an articulating surface which is either fully conforming or fully non-conforming relative to the head of the humeral implant. A fully conforming articulating surface has the same spherical radius as the humeral head, and thereby allows relative rotational movement therebetween. However, with a fully conforming articulating surface, the periphery or edge of the articulating surface is loaded by the head of the humeral implant when the arm is moved to a large degree relative to the upper body (such as when the arm is placed over the head). This edge loading at the periphery of the articulating surface may result in permanent deformation of the glenoid implant over a period of time.
A glenoid implant having an articulating surface which is fully non-conforming relative to the humeral head allows both rotation and a limited extent of translation of the humeral head relative to the glenoid implant. Such a fully non-conforming articulating surface normally has a spherical radius which is greater than the spherical radius of the humeral head, thereby allowing the rotational as well as rolling and translational movement therebetween. However, such a fully non-conforming articulating surface may result in increased contact pressures between the humeral head and glenoid implant because of the relatively small surface area which is in contact therebetween at any point in time. Such increased contact pressures also may result in permanent deformation of the humeral head and/or articulating surface on the glenoid component over a period of time.
Also, U.S. Pat. No. 4,106,130 provides a glenoid or scapular implant having an articulating surface with a compound concave bearing surface including a cupped area bounded by an annular trough. The curvature of the cupped area and the transverse curvature of the troughed area each are no greater than the curvature of the rounded convex bearing surface of the humeral articular surface. The cupped area and the annular trough adjoin each other at an obtuse angle. Accordingly, the ball of the humeral component can rock or move away from the cupped area and into the troughed area such that loading occurs at the edge between the cupped area and the troughed area. Such edge loading at the periphery of the cupped area may result in physical damage to the glenoid implant over a period of time.
What is needed in the art is a glenoid implant which allows rotation of the humeral head relative to the articulating surface during normal movements of the arm relative to the upper body, and which allows a limited extent of rolling and translational movement of the humeral head relative to the glenoid implant during more extreme movements of the arm relative to the upper body.